This invention relates to a low noise, high stability FM transmitter suitable for use in communication or as a test instrument.
One type of prior art FM transmitter has utilized a pair of varacter diode frequency sources each operating at a different carrier frequency, for example, 595 MHz and 525 MHz. One source is frequency modulated with the baseband signal, while the other is frequency modulated with the baseband signal inverted. The two modulated carriers are mixed to provide a difference frequency, in this case, a 70 MHz output. The present inventors were not satisfied with the operating characteristics of this type of transmitter, particularly the noise level. They have conceived an improvement which provides better noise characteristics and frequency stability, as well as good linerity.